Hybrid automobiles comprising a combination of an electric motor and an internal combustion engine, or electric motors that are employed in electric automobiles that are driven solely by an electric motor are driven by means of an inverter device. This inverter device drives the electric motor with a prescribed torque and frequency obtained by converting DC to AC. This inverter device is incorporated in the automobile and reduction in size thereof is desired, in order to ensure sufficient space within the vehicle to accommodate the driver and passengers etc.
The temperature of the switching elements of the inverter constituting the inverter device fluctuates considerably in accordance with the operating environment of the automobile: in particular, in the case of a hybrid automobile, the switching elements of the inverter reach a high temperature due to the effect of generation of heat by the internal combustion engine. In addition to such ambient temperature, the switching elements in the inverter are also raised in temperature due to the effect of steady losses caused by the current flowing in the switching elements themselves, and switching losses due to being turned on and off: if a certain temperature is exceeded, breakdown may even be caused.
In order to avoid breakdown of the switching elements, the inverter may be cooled or the temperature of the switching elements or inverter may be measured and the torque or switching frequency controlled accordingly. In order to improve inverter cooling, large heat radiating fins must be employed: however, if large heat radiating fins are employed, weight and volume are both increased. On the other hand, if the switching frequency is lowered, the ripple current is increased, which may cause the capacitor to generate heat or adversely affect controllability.
In view of these circumstances, generation of heat by the switching elements is ultimately suppressed by reducing the current flowing in the switching elements themselves by controlling the electric motor torque. For this reason, in some cases, it is arranged that, when the switching elements of the inverter exceed a prescribed temperature, torque control is initiated, in which torque control is applied proportional to the rise in temperature and rate of temperature change, so as to suppress generation of heat by the switching elements and avoid breakdown of the switching elements. An example of this is disclosed in Laid-open Japanese Patent Application No. H. 10-210790 (hereinbelow referred to as Patent Reference 1).
Also, the rate of temperature rise of the power transistor module may be detected, and action to protect the power transistor module performed if the rate of temperature rise exceeds a prescribed value. An example is found in Laid-open Japanese Patent Application No. H. 9-140155 (hereinbelow referred to as Patent Reference 2).
However, although Patent References 1, 2 describe applying a torque restriction when the switching elements exceed a prescribed temperature, no consideration was given to what amount of torque restriction would be suitable, depending on circumstances.
For example, if the amount of torque restriction is made proportional to the temperature of the switching elements, a proportionality constant that assumes a maximum ambient temperature should be employed for this proportionality constant. This is because, in order to reliably prevent breakdown of the switching elements, it is necessary to determine the amount of torque restriction assuming that the ambient temperature is in the highest condition. Consequently, in a condition in which the ambient temperature is low, a torque restriction is applied that is greater than necessary, which may result in uncomfortable handling when driving.
In order to minimize the amount of torque control, it is necessary to set the proportionality constant in accordance with the ambient temperature and, in order to achieve this, calibration for setting the proportionality constant, or introduction of a system for measuring the ambient temperature, is considered to be necessary. This complicates control and gives rise to the problems of lowered reliability or increased inverter volume and cost.
An object of the present invention is to provide an inverter device, electric automobile in which this inverter device is mounted, and hybrid automobile in which this inverter device is mounted wherein the amount of torque control needed for preventing rise in temperature of the switching elements can be minimized, wherein a higher power density of the inverter can be achieved by improving current throughput capability, and wherein handling comfort when driving can be improved and control simplified.